The present invention relates to photoconductive devices and methods of operating same and more particularly to such devices and methods used in imaging devices, photocells, etc., represented by image pick-up tubes, one-dimensional image sensors, two-dimensional image sensors and so on.
Conventionally, photosensors, one-dimensional image sensors, two-dimensional image sensors, image pick-up devices, etc., are generally known as photoconductive devices comprising at least a transparent conductive film and a photoconductive layer superimposed in order on a transparent substrate, as disclosed, for example, in Japanese Patent Publications JP-B-55-14554 and JP-B-59-26154 and Japanese Patent Publications JP-A-49-24619 and JP-A-63-304551.
Generally, the electrical characteristics of a photoconductive film depend on ambient temperature, so that the characteristics of the photoconductive device change depending on the temperature of the photoconductive layer in use. Especially, in a photoconductive device which uses an amorphous semiconductor which mainly includes selenium as a photoconductive film, a dark current and an after image increase as the temperature increases. Long time use of the photoconductive device at high temperature crystallizes the photoconductive film partially to deteriorate the image quality and thus reduce its service life. Conversely, if the temperature used is too low, the sensitivity is likely to decrease, the lag is likely to increase, etc. Thus, the photoconductive device is preferably used under conditions where its performance is best, which is obtained by controlling the temperature of the photoconductive film. For this reason, a method of operating a photoconductive device calls for cooling a target of the image pick-up tube with a thermoelectric cooling device in order to use the photoconductive device in its best state.